Noninfectious Skeletal Disorders in Broilers
These deformities often are seen as distinct flock problems. The most common abnormalities are seen in the distal limb and involve lateral or medial deviation and/or external rotation. Valgus or varus deviation of the intertarsal joint is the most common deformation in broiler chickens. Males are more commonly affected. The pathogenesis of the leg deformation is not well defined. Deformity may be a consequence of rickets at a younger age (see below). Poor mineralization of the bone, as in rickets, increases the potential for deformation and therefore the incidence and severity of deformities. Bone deformities may also be due to chondrodystrophy secondary to nutritional deficiencies (eg, choline, biotin, pyridoxine, folic acid). In breeds predisposed to deformities, the incidence may be reduced by slowing the growth rate via feed restriction or lighting programs. Rotated tibia has been a major problem in turkeys and a minor problem in Leghorns and guinea fowl. Excessive external tibial rotation occurs during development, but the pathogenesis is not well understood. Genetic, nutritional, and management factors are thought to be involved. The lack of bone angulation differentiates rotated tibia from valgus and varus deformation.
Vertebral deformities and/or displacements (spondylopathies) such as lordosis and kyphosis are common in thoracic vertebrae, particularly at the level of the free thoracic vertebra. Spondylolisthesis (“kinky back”) is a developmental disorder resulting in rotation of the free thoracic vertebra with ventral displacement of the anterior end and over-riding of the posterior end, causing spinal cord compression and posterior paralysis. Spondylolisthesis is the most common vertebral column deformity, but incidence is low in most broiler flocks. Growth rate and conformation influence this developmental disorder.
Dyschondroplastic lesions are masses of avascular cartilage extending from the growth plate into the metaphysis and are attributed to the failure of chondrocyte differentiation. This results in a focal thickening of the growth plate and is most commonly seen in the proximal tibiotarsus (tibial dyschondroplasia). Dyschondroplasia can develop in other bones such as the proximal and distal femur and tarsometatarsus. The lesion in the proximal tibiotarsus can be associated with anterior bowing of the tibiotarsus and sometimes fractures distal to the plug of cartilage. Factors shown to influence the incidence and severity of dyschondroplasia include genetic selection, rapid growth, calcium:phosphorus ratio in feed, metabolic acidosis due to excess chloride in feed, acid/base imbalance, copper-deficient diet, Fusarium spp contamination of the feed, and exposure to dithiocarbamate fungicides and certain antibiotics such as salinomycin. The most common causes in modern broiler flocks may be marginal inadequacies in dietary calcium or a calcium:phosphorus imbalance. The disease is seen in broiler chickens, turkeys, and ducks.
Rickets develops in growing birds due to deficiency of vitamin D, calcium, or phosphorus (see Calcium and Phosphorus Imbalances and see Vitamin D 3 Deficiency) or by calcium:phosphorus imbalance. Inadequate nutrition or intestinal disease with malabsorption can be the cause of the mineral or vitamin deficiency or imbalance. In rickets, abnormal endochondral ossification with failure of mineralization leads to defective bone formation, flexible long bones with subsequent bone deformities (eg, varus, valgus), and fractures. Lesions are most prominent at sites of rapid growth where cartilage contributes most significantly to skeletal growth (eg, long bones, ribs). Thick and pliable ribs and/or knobs at the costochondral junction (rachitic rosary) due to thickened and flared metaphyses are classic gross lesions. Beaks and claws can be soft and pliable. Hypertrophy and hyperplasia of the parathyroid glands are present. Subclinical rickets with only marginal thickening of the growth plates is fairly common and often associated with poor performance of broiler chickens. Bone pathology and bone ashing to estimate calcium and phosphorus content are useful diagnostic tools.
Degenerative joint disease is seen mainly in the coxofemoral, femorotibial, and intertarsal joints of broilers and male turkeys near market weight. Reluctance to move, abducted legs, and lameness can be observed. The pathogenesis is not well defined, but osteochondrosis or cartilage lesions and genetic factors may be involved.